The Dirty Dozen (Invasive Exotic Plants)

Below is a list of the “Dirty Dozen” plants that have become nuisances and even a significant threat to the natural environment in this region. The ranking is based in part on lists created by federal, state and local forestry services, invasive plant councils and land preservation groups and in part because landowners and developers in this area continue to use some of these plants in residential and commercial landscapes.

You may find this list of invasive species surprising since it includes many species readily available in your local garden center. While no plant is inherently “bad,” the invasive species listed below face little or no competition for limited space nor do they face pressure from natural predators or disease and therefore can get quickly out of control and destroy local vegetation. Research has shown that 85% of invasive plants were brought in as ornamental or landscaping plants. Such is the case of the notorious Japanese import, Kudzu, which was originally introduced to the U.S. as a ground cover. And cover the ground, trees, shrubs Kudzu has since its introduction. (Note: Kudzu is not listed below because public awareness is high.)

Why does it matter whether a plant is invasive? Invasive plants ARE a threat because:

they have no natural controls to limit their growth
invasives compete with native plants for space, sunlight, nourishment and breakdown the natural communities
by replacing native plants, invasives can limit or reduce the quality of food and shelter available to wildlife
they can harbor fungi, pathogens, varmints and other organisms that can be harmful to native plants and animals
reduce biodiversity by taking over natural communities
can be highly expensive to remove

Just driving down just about any roadway in this area, you can pick out many areas where one or more of these invasive plants have established a foothold. And you might even find one or more of these invasive exotics growing in your own yard. For information about how to eradicate these plants, you can go to resources listed on this page. If you’d like more information about how you can help fight invasive plants in our area, you can contact the Lookout Mountain Conservancy, the Tennessee Exotic Plant Pests Council as two organizations that provide workshops, volunteer opportunities and other activities to increase awareness of invasive plant control methodologies.

This is just a partial listing of the most invasive exotic plants readily found in this area. For a complete listing of invasive plants in Tennessee and their native substitutes, go here. Just in case you don’t think you have any invasive plants in your garden, a complete listing includes commonly planted shrubs like Nandina (Heavenly Bamboo), Burning Bush (Euonymous), Japanese Barberry, and Mahonia, vines like Chinese and Japanese Wisteria, Periwinkle (Myrtle), and grasses like Zebra grass.

Trees	Dossier of The Dirty	Better Choices
1.Tree-of-heaven Ailanthus altissima	This extremely vigorous invasive is listed by the Lookout Mountain Conservancy as one of the greatest threats to native species in our area. It grows quickly on disturbed hillsides, crowding out all other growth by forming a dense canopy that does not allow sunlight to filter down. Tree of heaven resembles the sumacs and hickories, but is easily distinguished by the glandular, notched base on each leaflet (buttons) and large leaf scars on the twigs. It is extremely tolerant of poor soil conditions and can even grow in cement cracks. Trees are not shade tolerant, but thrive in disturbed forests or edges. Tree of heaven, native to Asia, was first introduced into North America in 1748 by a Pennsylvania gardener. It was widely planted in cities because of its ability to grow in poor conditions.	There are many suitable native trees and shrubs including Black Locust, Sumac, Black Walnut, White and Blue Ash.
2. Mimosa (Silk Tree) Albizia julibrissin	Mimosa is a small tree that is 10 to 50 feet in height, often having multiple trunks with delicate looking fernlike compound leaves. The tree flowers in early summer with showy, feathery, fragrant, pink flowers that develop in groups at the ends of the branches. Fruit are flat, 6 inch seed pods that develop in the late summer. Like Tree-of-heaven, Mimosa quickly invades any type of disturbed habitat. It is commonly found in old fields, stream banks, and roadsides. Once established, mimosa is difficult to remove due to the long lived seeds and its ability to re-sprout vigorously. Mimosa is native to Asia and was first introduced into the U.S. in 1745. It has been widely used as an ornamental.	If you are looking for a showy flowering tree, look no further than natives like Redbud (Cercis canadensis), Flowering Dogwood (Cornus florida) and Red Buckeye (Aesculus fava)
Shrubs
3. Autumn Olive (aka Oleaster, Silverberry) Elaeagnus umbellata	Autumn olive is a deciduous shrub from 3-20 feet in height and is easily recognized by the silvery, dotted underside of the leaves. Leaves are alternate and 1 inch wide. The small, yellowish flowers are abundant and occur in clusters near the stems become red, juicy, and edible fruit in late summer and fall. Autumn olive invades old fields, woodland edges, and other disturbed areas. It can form a dense shrub layer which displaces native species and closes open areas. Autumn olive is native to China and Japan and was introduced into North America in 1830. Since then, it has been widely planted for wildlife habitat, mine reclamation, and shelterbelts.	Plant instead Waxmyrtle (Morella (Myrica) cerifera) or red or black chokeberry (Photinia pyrifolia).
4. Privet Ligustrum sp	Privets are a grouping of shrubs native to Asia, Europe and North Africa. All privets are thick, evergreen shrubs that grows up to 20 feet in height. The trunks are usually multiple stemmed with many long, leafy branches, making dense thickets. Leaves are opposite, oval, up to 2 inches long, with a pointed apex and often with margins that are slightly rolled. The leaves are leathery and contain high levels of phenolic compounds that make them resistant to insects, herbivores and pesticide control. Flowering occurs in spring to summer, when very abundant, white flowers occur in clusters at the end of branches. Fruits are small dark purple to black berries that persist into winter. Japanese privet commonly forms dense thickets in fields or forest understories. It shades and out-competes many native species, and once established is very difficult to remove. Privet was introduced into the United States in the early 1800s. There are several privet species and they are very hard to distinguish. Japanese privet is sometimes set apart by the thickness and glossiness of the leaves.	Devilwood (Osmanthus americanus), Inkberry (Ilex glabra), Southern Waxmyrtle (Morella (Myrica) cerifera) or Possumhaw Viburnum (Viburnum nudum) are great understory shrubs that provide food and shelter to birds and other desirable wildlife.
5. Amur Bush Honeysuckle Lonicera maackii	This is just one of several exotic honeysuckles that have invaded this area which include Morrow’s, January Jasmine, Bell’s Bush and Tartarian. Exotic bush honeysuckles can rapidly invade and overtake a site, forming a dense shrub layer that crowds and shades out native plant species. They alter habitats by decreasing light availability, by depleting soil moisture and nutrients, and possibly by releasing toxic chemicals that prevent other plant species from growing in the vicinity. Exotic bush honeysuckles may compete with native bush honeysuckles for pollinators, resulting in reduced seed set for native species. In addition, the fruits of exotic bush honeysuckles, while abundant and rich in carbohydrates, do not offer migrating birds the high-fat, nutrient-rich food sources needed for long flights that are supplied by native plant species.	There are many native shrubs that provide screening including Witch Hazel (Hamamelis Virginia), Southern Bush Honeysuckle (Diervilla sessilifolia), Red or Black Chokeberry (Photinia pyrifolia), Winterberry (Ilex verticillata), American Beautyberry (Callacarpa americana), Flame Azalea (Rhododendron calendulaceum)
6. Japanese Spirea (aka Japanese meadowsweet) Spiraea Japonica	Japanese spirea, also called Japanese meadowsweet, is a perennial, deciduous shrub that grows to 4 or sometimes 6 feet in height and about the same in width. It can be found in many forms and varieties at nurseries. It has slender erect stems that are brown to reddish-brown, round in cross-section and sometimes hairy. The leaves are generally egg-shaped, 1-3 inches long, have toothed margins and alternate along the stem. Clusters of attractive, rosy-pink flowers are borne at the tips of branches. Seeds, measuring about 1/10 inch in length, are contained in small lustrous capsules. While lovely, Japanese spirea can rapidly take over disturbed areas. Growing populations creep into meadows, forest openings, and other sites. Once established, spirea grows rapidly and forms dense stands that out-compete much of the existing native herbs and shrubs. Seeds of Japanese spirea last for many years in the soil, making its control and the restoration of native vegetation especially difficult.	Ninebark (Physocarpus opulifolius), Summersweet (Clethra alnifolia) provide 3 seasons of interest to your garden with flowers, seeds, fall foliage and even winter bark in the case of Ninebark.
Vines
7. English Ivy Hedera helix	English ivy is an evergreen vine that can grow to 100 feet in length (or height). The leaves are dark-green and waxy with palmate veins. Leaf shape is variable, but commonly occurs as a 3-5 lobed leaf with a heart-shaped base. Flowering (maturity) is triggered by sunlight, such as when the vines climb into taller vegetation. In the late summer mature plants produce terminal clusters of greenish-yellow flowers. Fruits are black and fleshy. English ivy can invade woodlands, fields and other upland areas and is spread by runners. Seeds can also be spread by birds and you may find small ivy plants growing in your garden despite having no plants nearby. It can grow both along the ground, where it can displace native understory species, and in the tree canopy, often covering branches and slowly killing trees. Ivy vines can also strangle or girdle trees, stressing the tree and making it vulnerable to boring insects and disease. English ivy is native to Europe and was introduced into North America by early settlers for ornamental purposes. It continues to be widely planted as an ornamental and is stocked by many gardening centers.	For erosion control in shady areas, consider Allegheny Spurge (Pachysandra procumbens), a native Pachysandra, or any of the many native ferns.
8. Japanese Honeysuckle (Lonicera japonica)	Japanese honeysuckle is an evergreen to semi-evergreen vine that can be found either trailing or climbing to over 80 ft. (24 m) in length. Leaves are opposite, sessile, oval and 1 to 2.5 inches long. Flowering occurs from April to July, when showy, fragrant, tubular, whitish-pink to yellow flowers develop in the axils of the leaves. Fruits develop in the fall and are small, shiny black berries. Japanese honeysuckle invades a variety of habitats including forest floors, canopies, roadsides, wetlands, and disturbed areas. Japanese honeysuckle can girdle small saplings by twining around them, and it can form dense mats in the canopies of trees, shading everything below. A native of eastern Asia, it was first introduced into North America in 1806 in Long Island, NY. Japanese honeysuckle has been planted widely throughout the United States as an ornamental, for erosion control, and for wildlife habitat.	Native Coral or Trumpet Honeysuckle (Lonicera sempervirens), and Yellow Honeysuckle (Lonicera flava) are well-behaved climbers that attract hummingbirds and butterflies to your garden.
9. Wintercreeper (aka Climbing Euonymous) Euonymus fortunei	Not to be confused with its legitimate native family member, Hearts-a-bustin (Euonymous americanus), Wintercreeper is an evergreen, woody vine that invades forests throughout the eastern United States. The plant can be a small shrub, growing in mats along the forest floor to 3 feet in height, or a vine climbing trees to heights of 40-70 feet. The opposite leaves are dark green, oval, slightly toothed, glossy, thick, less than 1 inch long and often with silvery-white veins. The young stems are green, becoming light gray and corky with age. Flowers are inconspicuous, yellow-green, five petaled and develop in mid-summer. Plants usually only flower when climbing and almost never when trailing along the ground. Fruit are pinkish-red capsules that open to show orange seeds. Wintercreeper aggressively invades open forests, forest margins, and openings. The dense ground cover often resulting from an infestation can displace native understory species and restrict tree seedling establishment. Wintercreeper can also smother and kill shrubs and small trees. Wintercreeper is native to Asia and was first introduced into the United States in 1907 as an ornamental ground cover plant.	Partridgeberry (Mitchella repens) and Winterberry/Eastern Teaberry (Gaultheria procumbens) provide a year-round green ground cover and provide food for birds and small mammals.
Herbs (Plants)
10. Japanese Knotweed (aka Fleeceflower, Mexican Bamboo) Polygonum cuspidatum, Fallopia japonica	Japanese knotweed is an upright, shrublike, herbaceous perennial that can grow to over 10 feet in height. The base of the stem above each joint is surrounded by a membranous sheath, stems are smooth, stout and swollen at joints where the leaf meets the stem. Although leaf size may vary, they are normally about 6 inches long by 3 to 4 inches wide, broadly oval to somewhat triangular and pointed at the tip. The minute greenish-white flowers occur in attractive, branched sprays in summer and are followed soon after by small winged fruits. Seeds are triangular, shiny, and very small, about 1/10 inch long. Japanese knotweed can tolerate a variety of adverse conditions including full shade, high temperatures, high salinity, and drought. It is found near water sources, such as along streams and rivers, in low-lying areas, waste places, utility rights-of-way, and around old homesites. It can quickly become an invasive pest in natural areas after escaping from cultivated gardens.	The following shrubs and perennials have the feathery white spires when in flower and attract bees and butterflies: Summersweet (Clethra alnifolia) Goatsbeard (Aruncus dioicus), Virginia Sweetspire (Itia virginica)
11. Purple Loosestrife (Lythrum salicaria)	Purple loosestrife is a perennial of European origin that can grow 3 – 4 feet tall, forming colonies 4 feet or more in width with numerous erect stems growing from a single woody root mass. The stems are reddish-purple or red to purple and square in cross-section. The flowers are reddish purple that are clustered tightly on a spike. Many states have declared purple loosestrife a noxious weed and banned its sale because of its propensity to spread and take over sensitive wetlands. These large monotypic stands of loosestrife threaten native plants and wildlife by eliminating their food sources and can even impede irrigation systems. This plant of European origin hitchhiked in ship ballasts and was also brought as a medicinal herb.	There are many native perennials with purple flower spikes including Obedient Plant (Physostegia virginiana), and Blazing Star (Liatris sp.)
Grasses
12. Cogongrass (aka Japanese Bloodgrass) Imperata cylindrica	This aggressive invader of natural and disturbed areas throughout this area displaces natural grasses, decreases tree seedling growth and alters fire regimes and intensity. Cogongrass appears as a bright green perennial grass that grows 2-6 feet tall in dense stands, crowding out other plant species. The leaves are flat and have saw-like edges lined with sharp microscopic silica crystals. The roots are sharp pointed white barbwire-life rhizomes that branch rapidly and readily, going through roots of adjacent plants. Flowers appear in late winter through May, and are formed on showy feathery spikes. White/silvery seeds are fluffy like dandelions and are dispersed widely in the wind. One variety, called “Red Baron” is claimed to be less aggressive, but there is no proof of this claim. Cogongrass usually appears on poor, acidic soils, doing best at a pH of around 4.7. Tennessee has declared it a noxious weed.	Replace with Red Switch Grass (Panicum virgatum), River Oats, or side oats grama (Bouteloua certipendula)